Silicon wafer is a main production raw material on semiconductor and photovoltaic industrial chains. MWS (Multi-Wire-Slicing) is a process for cutting brittle and hard materials (such as silicon ingot). Different from conventional inner diameter and outer diameter sawing method, its principle is that a high-speed moving metal slicing wire (typically a steel wire) drives the free slurry (or fixed abrasive, which has the higher production efficiency, but the cost is high, too) attached to it to grind silicon ingots, thereby achieving the purpose of slicing. In the whole slicing process, the steel wire, guided by a wire roller, form a wire net on the main wire roller, the feeding of the work piece to be processed is realized through the movement of the workbench, and hundreds of slices may be cut simultaneously. Compared with the existing slicing methods, it has remarkable advantages, such as high efficiency and high accuracy.
Currently thin silicon wafers with a large area (30 cm×30 cm) can be produced by MWS method, but as it is non-rigid slicing, inevitably the metal slicing wires are deformed and consequently generate instantaneous impact on the cut silicon wafers continuously in the slicing process. Meanwhile, the cooling of the steel wires in the kerf should also be considered. Technically speaking, it is rather difficult to further reduce the thickness of existing large-size silicon wafers, increase the slicing thickness of silicon wafers, control slicing loss and realize cost-effective and efficient slicing.
In the recent years, to address the application requirements on silicon wafer slicing technology, new process are searched at home and abroad in addition to the research on MWS. One of them is WEDM (Wire cut Electrical Discharge Machining). In this method, the wire electrodes typically are copper wires or molybdenum wires. Belgian Leuven University researched the slicing of silicon wafers by adopting low-speed WEDM. Japanese Okayama University researched the slicing processing of monocrystalline silicon rods by adopting WEDM and using deionized water as the working solution, and developed multi-wire electrical discharge slicing principle prototype (composed of a plurality of independent wire feeding systems). The basis of this method is that the monocrystalline silicon ingots formed by oriented growth has very low resistivity (0.01 Ω·cm), making the slicing of silicon ingots by WEDM technology possible. The total thickness variation (TTV) and warp of silicon wafers obtained by WEDM are almost the same as the results of MWS.
The loss of silicon material caused by the kerf is about 250 μm, equivalent to the value obtained from MWS. Nonetheless, in the foregoing relevant research, deionized water is used as the working medium, so the discharge energy is large and there is obvious heat affected zones on the surface of the silicon wafers and the processing efficiency is not high (at present, the highest slicing efficiency <100 mm2/min). Furthermore, the residue of metal elements on the surface of the silicon wafers is not considered. For this reason, it still can not compatible with solar energy silicon wafer cell manufacturing process.
Nanjing University of Aeronautics and Astronautics adopted an electric spark/electrolysis combined processing method to research the integration of slicing and texturing of solar energy silicon wafers (invention patent No.: ZL200710025572.5). Both heat affected zones and metal element residue are effectively controlled and single-wafer slicing efficiency is raised significantly. However, during MWS of silicon wafers, the discharge current is too large and wires are prone to breaking. As a result, this method can not be applied in MWS technology.
American Applied Materials, Inc. researched the planarization of substrate by electrochemical mechanical grinding method (invention patent No.: ZL02803505.4), and realized a method for planarizing substrate surface by a low contact pressure between the substrate and the grinding device.